Process for the production of polyurethane lens

ABSTRACT

The present invention relates to a process for the production of a polyurethane lens, which comprises 
     the step (a) of providing a polyisocyanate compound and two or more polythiol compounds which have different reaction rates with the polyisocyanate compound and adding an alkyltin halide compound to the above polyisocyanate compound, and, after the step (a), 
     the step (b) of mixing the polyisocyanate compound and the two or more polythiol compounds together with the alkyltin halide compound of the general formula (I) to allow them to react, and obtaining a polyurethane lens.

This application is a divisional application of Ser. No. 08/802,837,filed Feb. 18, 1997, which is a divisional of application Ser. No.08/559,632, filed Nov. 20, 1995 and reissued as U.S. Pat. No. 5,635,580.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for the production of apolyurethane lens.

2. Description of the Prior Art

It is disclosed in JP-A-63-130614 that, in the production of apolyurethane lens by reacting a polyisocyanate compound with a polythiolcompound, a tetrafunctional poly t hid compound such as pentaerythritoltetrakis(mercaptoacetate) or pentaerythritoltetrakis-(mercaptopropionate) is used in combination with a difunctionalpolythiol compound having two thiol groups, for example, for increasingthe crosslinking degree. It is also known that the reaction rate of onepolythiol compound with a polyisocyanate compound differs from that ofanother. As a method of producing a polyurethane lens free of an opticalstrain from these two or more polythiol compounds, generally, there isemployed a polymerization method in which the polymerization conditionsare suited to a polythiol compound having a higher reaction rate with apolyisocyanate compound, the initial polymerization temperature is setat a low temperature and the polymerization temperature is graduallyincreased with taking time.

However, in the above polymerization method in which the polymerizationconditions are suited to a polythiol compound having a higher reactionrate with a polyisocyanate compound, the initial polymerizationtemperature is set at a low temperature and the polymerizationtemperature is gradually increased with taking time, there is a problemin that the polymerization takes a long period of time to make theproduction efficiency poor. Further, when a lens having a large centralthickness and a large marginal thickness is produced, for example, frompentaerythritol tetrakis(mercaptoacetate) (to be referred to as PETMA.,hereinafter) which has a high reaction rate with a polyisocyanatecompound, the amount of PETMA increases, and the reaction heat generatedby the reaction with a polyisocyanate compound increases. It is hencedifficult to prevent the occurrence of an optical strain and striae bycontrolling the polymerization heat alone. There is therefore adisadvantage in that the yield of lenses per a polymerization furnace islimited in mass production.

SUMMARY OF THE INVENTION

The present invention has been made to overcome the above problems. Itis an object of the present invention to provide a process for theproduction of a polyurethane lens, which serves to decrease thepolymerization time for producing a polyurethane lens from apolyisocyanate compound and polythiol compounds and which permits theproduction of a lens free of an optical strain and striae with goodproducibility in producing plastic lens having a large central thicknessand a large marginal thickness.

The above object and advantages of the present invention is achieved bya process for the production of a polyurethane lens, which comprises thefollowing steps (a) and (b).

step (a): providing a polyisocyanate compound and two or more polythiolcompounds which have different reaction rates with the polyisocyanatecompound, and adding an alkyltin halide compound of the general formula(I),

(R₁)_(c)—Sn—X_(4−c)  (I)

wherein R₁ is methyl, ethyl, propyl or butyl, X is a fluorine atom, achlorine atom or a bromine atom and c is an integer of 1 to 3,

to the above polyisocyanate compound, and

step (b): after the above step (a), mixing the polyisocyanate compoundand the two or more polythiol compounds together with the alkyltinhalide compound of the general formula (I) to allow them to react, andobtaining a polyurethane lens.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will be detailed hereinafter.

The process of the present invention comprises the steps (a) and (b). Inthe step (a), a polyisocyanate compound and two or more polythiolcompounds which have different reaction rates with the polyisocyanatecompound are provided, and an alkyltin halide compound of the generalformula (I),

(R₁)_(c)—Sn—X_(4−c)  (I)

wherein R₁ is methyl, ethyl, propyl or butyl, X is a fluorine atom, achlorine atom or a bromine atom and c is an integer of 1 to 3,

is added to the above polyisocyanate compound.

In the above step (a), it is required to add the alkyltin halidecompound of the general formula (I) to a polyisocyanate compound. Thereason therefor is as follows. A polyurethane lens having a largecentral thickness and a large marginal thickness has an optical strainwhen the alkyltin halide compound of the general formula (I) is added toa mixture of a polyisocyanate compound with polythiol compounds, whilethe above problem can be overcome when the alkyltin halide compound isadded to a polyisocyanate compound before the polyisocyanate compound ismixed with polythiol compounds. Further, an alkyltin halide compound haspoor solubility in a polythiol compound, while the alkyltin halidecompound is easily soluble in a polyisocyanate compound. Therefore, theworking is facilitated.

The polyisocyanate compound used in the step (a) is not speciallylimited. It can be properly selected from polyisocyanate compoundsdisclosed in JP-A-60-199016, JP-A-57-136601, JP-A-63-46213 andJP-A-1-302202.

Specific examples of the polyisocyanate compound include polyisocyanatecompounds such as hexamethylene diisocyanate, isophorone diisocyanate,bis(isocyanatomethyl)cyclohexane, dicyclohexylmethane diisocyanate,cyclohexane diisocyanate, bis(isocyanatomethyl)bicycloheptane, xylylenediisocyanate, tetramethylxylylene diisocyanate, lysine estertriisocyanate, tris(isocyanatomethyl)cyclohexane, mesitylenetriisocyanate, bicycloheptane triisocyanate and hexamethylenetriisocyanate; allophanate-modified products, buret-modified productsand isocyanurate-modified products thereof; and adducts thereof withpolyols or polythiols. These polyisocyanate compounds may be used aloneor in combination. Other known isocyanate compounds may be used, whilethe isocyanate compound as a main component is required to bedifunctional or higher. Halogen atoms such as Cl or Br may be introducedinto a known aliphatic isocyanate compound having an aromatic ring inits molecule. As the polyisocyanate compound, particularly preferred arebis(isocyanatomethyl)cyclohexane, bis(isocyanatomethyl)bicycloheptaneand xylylene diisocyanate and mesitylene triisocyanate.

Examples of the combination of the two or more polythiol compounds usedin the present invention are as follows.

(i) A combination of a polythiol compound (S₁) which is disclosed inJP-A-60-199016 and known to have a high reaction rate with apolyisocyanate compound, with a polythiol compound (S₂) which isdisclosed in JP-A-63-46213 and known to have a relatively moderatereaction rate with a polyisocyanate compound.

The above, polythiol compound (S₁) includes ethylene glycoldithioglycolate, trimethylolpropane tris(thioglycolate) andpentaerythritol tetrakis(thiolycolate).

The above polythlol compound (S₂) includes pentaerythritoltetrakis(mercaptopropionate). trimethylolpropanetris(mercaptopropionate), trimethylolethane tris(mercaptopropionate),dichioroneopentyl glycol bis(mercaptopropionate) and dibromoneopentylglycol bis(mercaptopropionate).

(ii) A combination of the polythiol compound (S₃) of the general formula(II)

(R₁ )_(a)—C(CH₂OCOCH₂SH)_(b)  (II)

wherein R₁ is methyl or ethyl, a is an integer of 0 or 1, b is aninteger of 3 or 4, and a+b=4, or the formula (III),

with a polythiol compound (S₄) which is disclosed in JP-A-3-236386 andhas a relatively moderate reaction rate with a polyisocyanate compound.

The compound (S₃) which has the general formula (II) includestrimethylolpropane tris-(mercaptoacetate), trimethylolethanetris(mercaproacetate) and pentaerythritol tetrakis(mercaptoacetate).

The compound (S₃) which has the general formula (III) isdipentaerythritol hexakis-(mercaptoacetate).

The compound (S₄) includes dimercaptomethyldithian.

The amount ratio of the two or more polythiol compounds which havedifferent reaction rates with a polyisocyanate compound is not speciallylimited. The polythiol compound of the above general formula (II) or(III) which are known to have high reaction rates with a polyisoyanatecompound may be added in an amount of at least 20 mol % based on thetotal amount of the polythiol compounds.

Whether or not the two or more polythiol compounds have differentreaction rates with a polyisocyanate compound can be determined, forexample, by the following method.

(i) m-Xylylene diisocyanate is selected as a standard polyisocyanatecompound. One polythiol compound is mixed with m-xylylene diisocyanatein such amounts that the —NCO group/—SH group mixing ratio is 1.0. Themixture just after the mixture is prepared is measured for a viscosityat 20° C. and after 2 hours from the preparation of the mixture, themixture is measured for a viscosity at 20° C. Then, an amount ofviscosity change is determined.

(ii) Various polyisocyanate compounds are measured for amounts ofviscosity chance in the same manner as in the above (i).

(iii) Any two polythiol compounds are picked up, and compared withregard to the amount of viscosity change. When a difference between theamounts of viscosity change is at least 100 cps, the above two polythiolcompounds are taken as compounds which have different reaction rateswith a polyisocyanate compound.

The present inventors have found the following. In the step (a), whenthe alkyltin halide compound of the general formula (I) is added to apolyisocyanate compound, it can be easily dissolved in thepolyisocyanate compound, and a plastic lens having a large centralthickness and a large marginal thickness is remarkably free of anoptical strain and striae even if the amount of monomers is large forproducing such a plastic lens. Further, no precise temperature isrequired, and the polymerization time can be decreased. As a result,there is produced a remarkable effect that hundreds to thousands oflenses can be produced by the polymerization in a single polymerizationfurnace.

Examples of the alkyltin halide compound of the general formula (I)include moromethyltin trichloride, dimethyltin dicliloride, trimethyltinchloride, dibutyltindichloride, tributyltin chloride, tributylrinfluoride and dimethyltin dibromide. Although differing depending uponthe kinds of monomers and polymerization temperatures, the amount of thealkyltin halide compound based on the monomer mixture is 10 to 10,000ppm, preferably 50 to 8,000 ppm. When the amount of the alkyltin halidecompound is outside the range of from 10 to 10,000 ppm, it is difficultto adjust the polymerization rate, and produced lenses have opticalstrains and striae in many cases and are not suitable for optical use.It is preferred to add the alkyltin halide compound at a relativelylower temperature, for example, 0° C. to room temperature.

According to the present invention, the above-described effects areaccomplished by adding the alkyltin halide compound of the generalformula (I), and the effects of the present invention cannot beaccomplished by replacing the alkyltin halide compound with aconventional halogen-free tin compound, such as dibutyltin dilauratewhich is equally used as a catalyst in the field of production ofpolyurethane lenses.

The alkyltin halide compound may be further added to that polythiolcompound of the two or more polythiol compounds which has a lowerreaction rate with a polyisocyanate compound.

For obtaining the preferred properties of a lens in refractive index andprocessability, the mixing ratio of the polyisocyanate compound and thepolythiol compounds is set such that the (—NCO group)/(—SH group) molarratio is preferably 0.9 to 1.2, particularly preferably 0.95 to 1.10.

Further, for improving a lens in weatherability, additives such as anultraviolet light absorbent and an antioxidant may be added as required.

The step (b) of the present invention will be explained hereinafter.

The step (b) is a step in which, after the step (a), the polyisocyanatecompound and the two or more polythiol compounds are mixed together withthe alkyltin halide compound of the general formula (I), and the mixtureis allowed to react to obtain a polyurethane lens.

The polymerization in the above step (b) is preferably carried out bydegassing the mixture and then casting the mixture into a mold formed ofmold members of glass or metal and gaskets of a resin. For improving thereleasability between the mold and the resin of the molded lens, themold may be treated with a mold releasing agent, or a mold releasingagent may be added to the monomers. Although differing depending uponthe kinds of monomers used, generally, the polymerization time is 3 to96 hours, and the polymerization temperature is 0 to 130° C.

The present invention will be further explained hereinafter withreference to Examples.

EXAMPLE 1

Step (a)

0.01 part by weight of dimethyltin dichloride (to be referred to as“DMTDCl” hereinafter) was mixed with 94 parts by weight of xylylenediisocyanate (to be referred to as “XDI” hereinafter), and the mixturewas stirred, at a room temperature of 15° C.

Step (b)

54 Parts of pentaerythritol tetrakis(mercaotoacetate) (to be referred toas “PETMA” hereinafter), 53 parts by weight of dimercaptomethyldithian(to be referred to as “DMMD” hereinafter) and 0.10 part by weight of amixture of dibutoxyethyl acid phosphate with butoxyethyl acid phosphatewere added to, and fully mixed with, the above mixture. In addition, amixture of PETMA with XDI (—NCO group/—SH group=1.0) was separatelyprepared as a model and measured for a viscosity just after thepreparation of the mixture and after 2 hours from the preparation toshow 45 cps and 450 cps, respectively, and the amount of viscositychange (ΔV₁) was 415 cps. Further, a mixture of DMMD with XDI (—NCOgroup/—SH group=1.0) was separately prepared and measured for aviscosity just after the preparation of the mixture and after 2 hoursfrom the preparation to show 12 cps and 13 cps, respectively, and theamount of viscosity change (ΔV₂) was 1 cps. The difference between (ΔV₁)and (ΔV₂) was 414 cps, and it was determined that PETMA and DMMD haddifferent reaction rates with XDI. Then, the above-obtained mixture wasdegassed under a pressure of 5 mmHg, cast into a mold formed of glassmold members and polyurethane gaskets, allowed to stand for a while,temperature-increased up to 120° C. over 12 hours, and heated at 120° C.for 3 hours, and the resultant lens was taken out of the mold. The moldhad been formed of an upper mold member of glass having a curvature of600 mm and a lower mold member of glass having a curvature of 120 mmsuch that the lens had a central thickness of 5 mm and a diameter of 75mm. In this Example, 200 lenses were produced by the polymerization in asingle polymerization furnace. The so-obtained lenses were evaluated,and Tables 1 and 2 show the results.

As shown in Tables 1 and 2, all the polyurethane lenses obtained in thisExample were free of cludiness and also free of striae and an opticalstrain.

The evaluation standards and methods were as follows.

Refractive index·Abbe's number: Measured with an Abbe refractometer 2Tsupplied by ATAGO CORP. at 20° C.

Transparency: Lenses were visually observed under a fluorescence lamp ina dark place, and those free of the cloudiness of the lens and theprecipitation of a translucent substance were rated as (A), and thosehaving a distinct precipitation were rated as (X).

Heat resistance: Measured with a TMA apparatus supplied by Rigakusha. Achart was prepared by a TMA method (penetration method) using a pressurepin having a diameter of 0.5 mm at a temperature increase rate of 10°C./min. under a load of 10 g, and lenses were evaluated on the basis ofthe resultant chart peak temperatures.

Weatherability: Lenses were set in a weatherometer equipped with asunshine carbon arc lamp, and allowed to stand for 200 hours. Then, thehue before the test and the hue after the test were compared. Concerningevaluation standards, those which had undergone almost no change wererated as (A), those which had slightly turned yellowish were rated as(B) and those which had turned yellowish were rated as (X). Theevaluation of a lens as (B) means that the lens slightly turns yellowishbut has no problem in practical use.

Optical strain: Lenses were visually observed with a strain scope. Thosefree of a strain were rated as (A), and those having strains were ratedas (X).

Striae: Lenses were visually observed by the Schlielen method. Thosealmost free of striae were rated as (A), and those having striae wererated as (X).

Examples 2-21

Polyurethane lenses were obtained in the same manner as in Example 1except that the composition of a polyisocyanate compound and polythiolcompounds was changed as shown in Tables 1 and 2. Tables 1 and 2 alsoshow the difference (|ΔV₁−ΔV₂|) between the amounts of viscosity change,ΔV₁ and ΔV₂, of the two polythiol compounds. As shown in Tables 1 and 2,all the polyurethane lenses were free of cloudiness, striae and anoptical strain.

Comparative Examples 1-4

Polyurethane lenses were obtained in the same manner as in Example 1except that the alkyltin halide compound of the general formula (I) wasreplaced with dibutyltin dilaurate as shown in Table 3. As shown inTable 3, the so-obtained polyurethane lenses had an optical strain andstriae to a. great extent.

Comparative Example 5

Plastic lenses were obtained in the same manner as in Example 1 exceptthat DMTDCl was added after XDI, PETMA and DMMD were uniformly mixed. Asshown in Table 3, most of the so-obtained polyurethane lenses showed anoptical strain and striae.

TABLE 1 Viscosity difference Compositional Catalyst/ Ex- |ΔV₁-ΔV₂| ratioamount ample Monomers (cps) (parts by weight) (ppm) 1 XDI 414 94 DBTDCl/PETMA.DMMD 54.53 1,200 2 H₆XDI 414 97 DMTDCl/ PETMA.DMMD 54.53 1,000 3IPDI 414 111 DMTDCl/ PETMA.DMMD 54.53 1.500 4 XDI 449 94 DBTDCl/DPETMA.DMMD 116.53 200 5 H₆XDI 449 97 DMTDCl/ DPETMA.DMMD 116.53 1,000XDI: xylylene diisocyanate DMMD: dimercaptomethyl dithian IPDI:isophorone diisocyanate H₆XDI: bis(isocyanatomethyl)cyclohexan DBTDCl:dibutyltin dichlorlide DMTDCl: dimethyltin dichlorlide PETMA:pentaerythritol tetrakis-(mercaptoacetate) DPETMA: dipentaerythritoltetrakis-(mercapto- acetate) PETMP: pentaerythritoltetrakis-(mercaptopropionate) DMMTP:2,3-dimercaptoethylthio-1-mercaptopropane DBTL: dibutyltin dilaurateRefractive Heat Trans- index/Abbe's resis- Weather- Example parencynumber tance ability Striae Strain 1 A 1.64/34 110 B A A 2 A 1.60/42 118A A A 3 A 1.60/42 141 A A A 4 A 1.63/34 112 B A A 5 A 1.60/42 121 A A A

TABLE 2 Viscosity difference Compositional Catalyst/ Ex- |ΔV₁-ΔV₂ ratioamount ample Monomers (cps) (parts by weight) (ppm) 6 IPDI 449 111DMTDCl/ DPETMA.DMMD 116.53 1.500 7 XDI 411 94 DBTDCl/ PETMA.PETMP 54.51100 8 H₆XDI 411 97 DMTDCl/ PETMA.PETMA 54.61 1,000 9 XDI 412 94 DBTDCl/PETMA.DMMTP 54.43 200 10 H₆XDI 412 97 DMTDCl/ PETMA.DMMTP 54.43 1,500 11H₆XDI 447 97 DMTDCl/ DPETMA.DMMTP 116.43 1,500 XDI: xylylenediisocyanate DMMD: dimercaptomethyl dithian IPDI: isophoronediisocyanate H₆XDI: bis(isocyanatomethyl) cyclohexane DBTDCl: dibutyltindichloride DMTDCl: dimethyltin dichloride PETMA: pentaerythritoltetrakis-(mercaptoacetate) DPETMA: dipentaerythritol tetrakis-(mercapto-acetate) PETMP: pentaerythritol tetrakis-(mercaptopropionate) DMMTP:2,3-dimercaptoethylthio-1-mercaptopropane DBTL: dibutyltin dilaurateRefractive Heat Trans- index/Abbe's resis- Weather- Example parencynumber tance ability Striae Strain 6 A 1.60/43 144 A A A 7 A 1.80/35 100B A A 8 A 1.56/45 103 A A A 9 A 1.64/34 100 B A A 10 A 1.60/42 103 A A A11 A 1.60/42 105 A A A

TABLE 3 Compositional Catalyst/ Comparative ratio amount ExampleMonomers (parts by weight) (ppm) 1 XDI 94 DBTL/ PETMA.DMMD 54.53 200 2XDI 94 DBTL/ PETMA.PETMP 54.61 500 3 IPDI 111 DSTL/ DPETMA.DMMD 116.535,000 4 H₆XDI 97 DMTL/ PETMA.DMMTP 54.43 5,000 5*  XDI 4 DBTDCl/PETMA.DMMD 54.53 200 XDI: xylylene diisocyanate DMMD: dimercaptomethyldithian IPDI: isophorone diisocyanate H₆XDI: bis (isocyanatomethyl)cyclohexane DBTDCl: dibutyltin dichloride DMTDCl: dimethyltin dichloridePETMA: pentaerythritol tetrakis-(mercaptoacetate) DPETMA:dipentaerythritol tetrakis-(mercapto- acetate) PETMP: pentaerythritoltetrakis-(mercaptopropionate) DMMTP:2,3-dimercaptoethylthio-1-mercaptopropane DBTL: dibutyltin dilaurateRefractive Heat Trans- index/Abbe's resis- Weather- CEx. parency numbertance ability Striae Strain 1 A 1.64/34 110 B X X 2 A 1.63/34 112 B X X3 A 1.60/42 144 A X X 4 A 1.60/42 103 A X X 5*  A 1.64/34 110 B X X*Comparative Example 5 differed from Examples 1 to 11 in that DBTDCl wasadded after XDI, PETMA and DMMD were mixed. CEx = Comparative Example

According to the process for the production of a polyurethane lens,provided by the present invention, polyurethane lenses remarkably freeof an optical strain can be mass-produced even if the polyurethanelenses have a large central thickness and a large marginal thickness.

What is claimed is:
 1. A process for mass-producing a polyurethaneophthalmic lens comprising the steps of: (A) adding an alkyltin halidecompound of the formula (I), (R₁)_(c)—Sn—X_(4−c)  (I) wherein R₁ ismethyl, ethyl, propyl or butyl, X is a fluorine atom, a chlorine atom ora bromine atom and c is an integer of 1 to 3, to a polyisocyanatecompound in an amount necessary to mass-produce the ophthalmic lens toform a first mixture; (B) subsequent of step (A), adding two or morepolythiol compounds in an amount necessary to mass-produce theophthalmic lens to the mixture of step (A) to form a second uniformmixture, said polythiol compounds having different reaction rates withthe polyisocyanate compound; (C) subsequent to step (B), degassing thesecond mixture; (D) subsequent to step (C), pouring the resultingmixture of step (C) into a number of ophthalmic lens molds necessary tomass-produce the ophthalmic lens, and (E) heating the filled lens moldsof step (D) in the same single polymerization furnace and allowing themixture to react to obtain the ophthalmic lens.
 2. The process of claim1, wherein one or more additional isocyanate compounds are used.
 3. Theprocess of claim 1, wherein the polyisocyanate compounds is, or thepolyisocyanate compounds are, selected from the group consisting of (i)a polyisocyanate compound selected from the class consisting ofhexamethylene diisocyanate, isophorone diisocyanate,bis(isocyanatomethyl)cyclohexane, dicyclohexylmethane diisocyanate,cyclohexane diisocyanate, bis(isocyanatomethyl)bicycloheptane, xylylencdiisocyanate, tetramethylxylylene diisocyanate, lysine estertriisocyanate, tris(isocyanatomethyl)cyclohexane, mesitylenetriisocyanate, bicycloheptane triisocyanate and hexamethylenetriisocyanate; (ii) a modified product selected from the classconsisting of an allophanate-modified product, a buret-modified productand an isocyanurate-modified product of the polyisocyanate compound (i),and (iii) an adduct of the compound (i) or (ii) with a polyol or apolythiol.
 4. The process of claim 1, wherein one of the two or morepolythiol compounds is a polythiol compound (S₁) selected from the groupconsisting of ethylene glycol dithioglycolate, trimethylolpropanetris(thioglycolate) and pentaerythritol tetrakis(thioglycolate) and theother polythiol compound is, or the other polythiol compounds are,polythiol compound(s) (S₂) having a lower reaction rate with thepolyisocyanate compound than the polythiol compound (S₁).
 5. The processof claim 1, wherein one of the two or more polythiol compounds is apolythiol compound (S₃) of the following general formula (II) or thefollowing formula (III), (R₁)_(a)—C(CH₂OCOCH₂SH)_(b)  (II) wherein R₁ ismethyl or ethyl, a is an integer of 0 or 1, b is an integer of 3 or 4,and a+b=4,

and the other polythiol compound is, or the other polythiol compoundsare, polythiol compound(s) (S₄) which has or have lower reaction rate(s)with the polyisocyanate compound than the polythiol compound (S₃) of thegeneral formula (II) or the formula (III).
 6. The process of claim 5,wherein the polythiol compound (S₃) of the general formula (II) isselected from the group consisting of trimethylolpropanetris(mercaptoacetate), trimethylolethane tris(mercaptoacetate) andpentaerythritol tetrakis(mercaptoacetate).
 7. The process of claim 5,wherein the polythiol compound (S₄) is dimercaptomethyl dithian.
 8. Theprocess of claim 1, wherein the alkyltin halide compound of the generalformula (I) is selected from the group consisting of monomethyltintrichloride, dimethyltin dichloride, trimethyltin chloride, dibutyltindichloride, tributyltin chloride, tributyltin fluoride and dimethyltindibromide.
 9. The process of claim 8, wherein the alkyltin halidecompound is added to a monomer mixture in an amount of 10 to 10,000 ppmbased on the monomer mixture.
 10. The process of claim 1, wherein thealkyltin halide compound is further added to the polythiol compoundwhich has a lower reaction rate with the polyisocyanate compound, amongthe two or more polythiol compounds.
 11. The process of claim 1, whereinthe polyisocyanate compound and the polythiol compounds are used in an(—NCO group)/(—SH group) molar ratio of 0.9 to 1.2.
 12. The process ofclaim 1, wherein a mixture obtained in the step (a) is degassed beforecarrying out the step (b).
 13. The process of claim 1, wherein thereaction for polymerization is carried out for 3 to 96 hours at atemperature of 0 to 130° C.